Data transfer systems have been developed recently wherein a rather broadband transport channel is used for data transfer. In radio systems in particular, the trend seems to be shifting away from the use of a narrowband channel towards broadband channels. The first mobile telephone systems, for example, used narrowband channels but both a so-called third generation UMTS system being developed and a fourth generation system already in view use a broadband transport channel.
The use of a broadband channel provides several advantages over narrowband systems. Broadband channels enable higher capacity to be achieved more easily. In addition, it is impossible that a signal could fade completely in a broadband channel, as is the case in a narrowband channel since in a narrow frequency band fading often occurs simultaneously. Fading is caused by multipath signal propagation when a signal propagates from a transmitter to a receiver. The impulse response of a narrowband channel is thus a single impulse while the frequency response, i.e. the attenuation of a signal propagated through a channel as a function of frequency, is a constant. The frequency response of a broadband channel is not a constant because of the broad band but because a signal attenuates differently in different frequency bands. The channel can thus be considered to be frequency selective. A discrete Fourier transform (DTF) of the impulse response of the channel is no longer constant but different frequencies attenuate differently.
As data transfer rates increase, the need for reliable data transfer and as efficient utilization of the channel used as possible is constantly on the increase. In the known broadband systems, the transmission power is evenly distributed over the frequency band of the channel used. In order to compensate for fading, a solution has previously been used in an orthogonal frequency division multiple access OFDMA connection, wherein differently faded frequency bands are amplified in different ways.